1. Field of the Invention
The present invention relates to a reflective type IPS (in-plane switching) mode LCD (liquid crystal display), and the control of liquid crystal molecules therein.
2. Description of Prior Art
A liquid crystal display utilizes the optical and electrical anisotropy of liquid crystal molecules thereof in order to produce an image. The liquid crystal molecules have a particular passive orientation when no voltage is applied thereto. However, in a driven state, the liquid crystal molecules change their orientation according to the strength and direction of the driving electric field. A polarization state of incident light changes when the light transmits through the liquid crystal molecules, due to the optical anisotropy of the liquid crystal molecules. The extent of the change depends on the orientation of the liquid crystal molecules. Thus, by properly controlling the driving electric field, an orientation of the liquid crystal molecules is changed and a desired image can be produced.
The TN (twisted nematic) mode LCD was the first main type of LCD developed. Even though TN mode LCDs have been put into use in many applications, they have an inherent drawback that cannot be eliminated; namely, a very narrow viewing angle. By adding compensation films on TN mode LCDs, this problem can be ameliorated to some extent. However, the cost of the TN mode LCD is increased. Therefore, a totally different driving means called IPS (in-plane switching) was proposed as early as in 1974. Then in 1993, Hitachi Corporation filed its first US patent application concerning IPS, in which an IPS mode LCD was disclosed.
A conventional IPS mode LCD has an upper substrate, a lower substrate, and a liquid crystal layer interposed therebetween. The liquid crystal layer has a plurality of liquid crystal molecules which have a same orientation when not driven, this orientation being parallel to the substrates. Pixel electrodes and common electrodes are disposed on the lower substrate. When a voltage is applied to the electrodes, an electric field is generated between the electrodes. The electric field drives the liquid crystal molecules to rotate in a plane parallel to the substrates. This rotation results in a change in light transmission. In other words, the operation of the IPS mode LCD is such that the liquid crystal molecules rotate in a plane parallel with the substrates in order to fulfill optical switching. The IPS LCD has the important advantage of a wide viewing angle. In basic IPS mode LCDs, the pixel electrodes and common electrodes are each comb-shaped. The electric field of these LCDs in a driven state is along a certain direction. Thus these LCDs are referred to as single domain IPS LCDs. When the displayed image is viewed at different oblique angles, a viewer may notice a quite large color shift. When this single domain IPS technology is applied in a reflective type liquid crystal display, color shift is likewise a problem.
Referring to FIG. 8 and FIG. 9, these are schematic plan representations of a conventional reflective type IPS liquid crystal display disclosed in U.S. Pat. No. 6,577,368 issued on Jun. 10, 2003. The liquid crystal display comprises an upper substrate (not shown), a lower substrate 80, and liquid crystal molecules (not shown) interposed between the substrates. A plurality of gate lines 10 and data lines 70 are formed on the lower substrate 80, for transmitting a gate signal and a data signal respectively. TFTs (thin film transistors) are positioned at the intersections of the gate lines 10 and data lines 70. Each TFT comprises a gate electrode 11, a source electrode 71, and a drain electrode 72. Pixel electrodes 75 and common electrodes 20 are formed on the lower substrate 80, for generating a driving electric field. A lower alignment film 4 is disposed at the lower substrate 80, to control the alignment of the liquid crystal molecules.
Each of the pixel electrodes 75 and the common electrodes 20 is comb-shaped. Therefore when a voltage is applied to the pixel and common electrodes 75, 20, a parallel electric field mainly along a single direction is generated. The pixel electrodes 75 and the common electrodes 20 are made of an opaque material having high reflectance.
When the voltage is applied, the liquid crystal display is said to be in a driven state. The liquid crystal molecules have the same orientation due to the electric field. When the LCD display is viewed from an oblique angle, a color shift is seen. That is, the viewer sees a yellowish or a bluish image.
It is desired to provide a multi-domain reflective type IPS liquid crystal display which overcomes the above-described deficiencies.